1968 can be a picky yeast. I have had it drop completely out of suspension and stall with only a 1 degree temperature drop. It is really important with that yeast to maintain a steady temp, or better yet, a slowly warming ferment. I usually pitch that one around 65-67 and ramp it up to 70 or so over 7 days. I would try warming it up to 70-75F to try to get it to finish out.
Or, you can use 1098 next time, which will give you a similar flavor profile without being as finicky. The 1098 will ferment out drier though, so you have to account for it in recipe and process formulation.